sudo apt update
sudo apt install python3-venv python3-pip

mkdir myproject
cd myproject
python3 -m venv ./venv
source venv/bin/activate
(venv) ~/workdir/myproject$

pip3 install pyside6

$ python
Python 3.11.2 (main, Apr 28 2025, 14:11:48) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import PySide6
>>> 
(venv)

vi app.py

from PySide6.QtWidgets import QApplication, QWidget

import sys

app = QApplication(sys.argv)

window = QWidget()
window.show()

app.exec()

python3 app.py